δ¹⁵N of Nitric Oxide Produced Under Aerobic or Anaerobic Conditions From Seven Soils and Their Associated N Isotope Fractionations

Abstract

Measuring the nitrogen isotope compositions (δ 15 N) of nitric oxide (NO) from different sources helps to quantify the relative contributions of atmospheric NO x. Soil is one of the most important sources of atmospheric NO x , but only limited measurements on the δ 15 N of soil-emitted NO exist, hampering our ability to partition sources to air pollution. Here we conducted soil incubations to measure the δ 15 N-NO under defined aerobic or anaerobic conditions, favoring either nitrification or denitrification. Soils were collected from seven sites spanning three ecosystems in northern China (two agricultural, two forest, and three grassland sites). We found that the δ 15 N-NO and their associated N isotope fractionations were significantly different between anaerobic and aerobic conditions in seven soils. Under aerobic condition, the δ 15 N-NO ranged from −62‰ to −50‰ (averaged −56 ± 4‰), being significantly more negative (by 23‰) than those under anaerobic condition (−45‰ to −23‰, averaged −33 ± 7‰). The apparent N isotope fractionation for NO production under aerobic condition (15 ε aerobic ¼ 61 ± 3‰) was significantly higher (by 26‰) than under anaerobic condition (15 ε anaerobic ¼ 35 ± 6‰), with a small variability among ecosystem types. Our study demonstrates that the δ 15 N-NO from different soils are very different from fuel combustions (mainly from 0‰ to +20‰), supporting that measuring 15 N is a useful tool to partition the contributions of soil NO to atmospheric NO x. Our results also imply δ 15 N-NO produced by nitrification and denitrification distinctly different, as these two processes are dominant processes producing NO under aerobic and anaerobic conditions, respectively. Plain Language Summary Nitric oxide (NO) affects the atmosphere chemistry and NO itself a key component of air pollution and a precursor to other air pollutants like particulate matter and ozone. To reduce the air pollution, it is crucial to identify the atmospheric NO sources. Measuring 15 N natural abundance has been considered as a promising tool to identify different sources, but the measurement on δ 15 N for soil-emitted NO is limited, adding to the large uncertainties on NO partitioning. Here we measured the δ 15 N-NO emitted from three ecosystem types across seven sites (two agriculture, two forest, and three grassland sites) under defined anaerobic or aerobic conditions. We found that soil-emitted NO was 15 N-depleted (−62‰ to −23‰) related to the source from fossil fuel combustion (0-20‰), suggesting that δ 15 N can be used to separate these two sources. We also found that the δ 15 N-NO under aerobic condition (nitrification prevailing, −62‰ to −50‰) was much lower than under anaerobic condition (denitrification prevailing, −45‰ to −23‰), which provide a useful tool to better understand the relative contributions of different sources of NO. In sum, soil-emitted NO has their own different isotope fingerprint, depending on the soil oxygen conditions, and can be used to partition sources.

Formation Mechanisms and Source Apportionments of Nitrate Aerosols in a Megacity of Eastern China Based On Multiple Isotope Observations

“…The δ 15 N of NO x in the emission of coal combustion was relatively higher (17.9‰ ± 3.1‰; Felix et al., 2012; Heaton, 1990; Zong et al., 2022). The NO x emitted from nitrification and denitrification normally has lower δ 15 N values (−43.7‰ ± 15.9‰; Felix & Elliott, 2014; D. Li & Wang, 2008; D. J. Miller et al., 2018; Su et al., 2020; Z. Yu & Elliott, 2017). The nitrogen isotopic fractionation during the NO x (g) ‐NO 3 − (p) conversion was associated with air temperature and the nitrate formation pathways, thus it varied a lot in different seasons (Walters & Michalski, 2016).…”

“…For instance, the δ 15 N of NO x emitted from combustion processes, such as BB (1.8‰ ± 4.1‰) and fossil fuel combustion (0‰–20‰), have higher values comparing to other NO x emission sources (Elliott et al., 2009; Fibiger & Hastings, 2016). NO x is depleted in 15 N when derived from natural sources such as nitrification/denitrification in soil (<−20‰; Felix & Elliott, 2014; D. Li & Wang, 2008; D. J. Miller et al., 2018; Su et al., 2020; Z. Yu & Elliott, 2017). However, nitrogen isotopic composition of nitrate is also depended on the nitrogen isotopic fractionation (Δ 15 N) during the oxidation of NO x into atmospheric nitrate besides to the sources of NO x (Elliott et al., 2007; Vicars et al., 2013).…”

Increasing Nonfossil Fuel Contributions to Atmospheric Nitrate in Urban China from Observation to Prediction

“…S j represents the δ 15 N­(NO x ) of each source and is distributed with mean values (μ j ) and standard deviation (ω j ). δ 15 N signatures for the five NO x sources were assigned as follows: 17.9 ± 3.1‰ for coal combustion, − −7.2 ± 7.4‰ for transport oil combustion, ,− −17.9 ± 1.0‰ for natural gas, −1.3 ± 4.3‰ for biomass burning, ,− and −43.7 ± 15.9‰ for soil, − based on the literature. Here, 10,000 feasible solutions of f j were generated through the performed in the SIAR model.…”

Long‐Term Evolution of Particulate Nitrate Pollution in North China: Isotopic Evidence From 10 Offshore Cruises in the Bohai Sea From 2014 to 2019

“…The contributions of the other three sources of NO 3 − were higher in summer than in winter, suggesting that the emission intensities of these sources were relatively high in summer. Generally, the microbial processes that produce NO x refer to the nitrification and denitrification, which mainly occur in soil and are affected by temper- ature (Su et al, 2020). The summer season in North China is hot and wet, where the temperature and humidity are conducive to these microbial activities (Yu et al, 2021).…”